In adults, the lungs have been shown to be involved in the regulation of many circulating vasoactive and "neural hormone" substances. The development of the metabolic processes required for this regulation is for the most part unknown. A major portion of the metabolic functions of adult lungs have been studied in isolated, perfused lung preparations. The co-investigator has developed a system for perfusing lungs isolated either from newborn or from previable, non-breathing fetal rabbits. These preparations have been used as a tool to study pulmonary metabolic functions in fetal and newborn lungs. This data is compared and contrasted to work done in isolated perfused adult lung preparations. We have found that 5-hydroxytryptamine (5HT) is taken up by all three lung preparations. In contrast, prostaglandin E1 (PGE1) is not taken up as well by newborn and fetal lungs as by isolated adult lungs.